A motor vehicle normally has an internal-combustion engine having an output shaft provided with a large-diameter flywheel formed on its edge with a periphery of drive teeth. A heavy-duty electrical motor is mounted adjacent to this flywheel and itself drives a small-diameter pinion which can mesh with the teeth of the flywheel, achieving a considerable speed reduction, to turn over the engine so that it can start. It is normally necessary to displace the pinion parallel to its axis of rotation into and out of mesh with the teeth of the flywheel, so that during normal operation of the engine the starter motor is not coupled to it.
Several systems are known for achieving this axial displacement of the starter gear or pinion. The most common one is of the so-called BENDIX type wherein the pinion is provided with an off-center weight and is carried between a pair of abutments on a compound thread on the output shaft of the starter motor. On the starter motor commencing rotating its output shaft the imbalanced gear therefore is screwed axially toward the flywheel until it meshes therewith. Once this gear has been screwed all the way to the end of its travel it stops and torque is transmitted between the output shaft of the starter motor and the flywheel. Once the internal-combustion engine catches, the flywheel rotates at a peripheral speed greater than that of the pinion so that the pinion is effectively screwed back down into the shaft and out of engagement with the flywheel. Such an arrangement has the advantage of considerable simplicity, yet has a relatively short service life due to the considerable shock that it must withstand as the pinion moves axially rapidly into mesh with the flywheel teeth, and as the pinion reaches either end of its travel.
For this reason systems have been suggested whereby a simple heavy-duty solenoid is effective through a pivotal fork to displace the starter gear axially on axially extending splines on the output shaft of the starter motor. The solenoid can displace the starter gear forwardly, and a heavy-duty spring urges it continuously backwardly so that the solenoid is deenergized and the starter gear is moved out of mesh with the flywheel teeth. Normally an overruning as one-way clutch is provided between the starter gear and the starter motor to prevent the internal-combustion engine from back-driving the starter motor. This system has the disadvantage that the solenoid, once again frequently subjected to severe shocks, has a short service life.
Another classic problem with these known types of starters is that as the starter gear is moved axially to mesh with the flywheel teeth the teeth of the gear and the teeth of the flywheel come into axial contact with each other, clashing and occasionally preventing meshing of the two. Mounting the pinion for limited angular motion relative to the output shaft by means of a heavy-duty spring overcomes this disadvantage, but once again introduces a failure-prone element into the mechanism.
It has been suggested in British Pat. No. 1,243,920 to avoid the use of a massive solenoid by employing a rotary control motor that drives a worm engaged by a nut connected directly or indirectly to the starter-motor pinion. The disadvantage of this system is that the speed both axially forwardly into engagement with the teeth of the flywheel and axially backwardly away from the flywheel due to the simple mechanics of the system is substantially the same and is, in fact, very slow. Thus the user of such an arrangement must inherently deal with a relatively sluggish starter, one that cannot be actuated and actuated immediately again, but wherein some time must elapse between actuations, for the control motor to screw back the starter-motor gear.